Typical radar, electromagnetic detection and/or surveillance schemes of the past and present have employed only a single band or a single range of electromagnetic energy bands.
Such systems are typically complex. This tends to discourage the development of schemes and systems operating in more than a single range or a single set of bands of electromagnetic energy.
Prior art millimeter radar systems further typically operate with only one feed at the focal point of an antenna. For purposes herein, a feed is generally considered to be a source of electromagnetic radiation capable of receiving the same. Exceptions to this approach are known, (e.g., phased arrays, Luneberg lens antennas, multiple or extended feeds, etc.), but they are generally either very expensive or they result in degraded performance.
On Feb. 27, 1984, however, the Applicant herein applied for a patent ("Wide Angle Multi-Mode Antenna," Ser. No. 584,273) on a radar antenna which did utilize two separate feeds in a common aperture arrangement operating at about 95 GHz. This system permits the operation of the antenna with each beam independently, or in concert.
Using two kinds of beams operating in the same frequency range provides enhanced operational flexibility. However, such a system remains subject to diffraction, a fundamental resolution limitation. This diffraction in any such system remains directly proportional to the operating wavelength, thereby limiting the resolution of microwave and millimeter wave systems.
Thus, the resolution attainable with millimeter radar, while better than that with lower frequency radar, still remains several orders of magnitude coarser than attainable with infrared systems operating in either the 3-5 micrometer or the 8-12 micrometer wavelength region. These regions are often chosen for infrared systems because the Earth's atmosphere is relatively transparent. Furthermore, infrared systems can operate passively, i.e., they do not need to flood a target actively with radiation in order to observe the reflected energy, as do radar systems. Rather, passive infrared systems detect heat energy which is directly emitted by the target. This passive operation offers concealment during military operations, and is not susceptible to radar jamming techniques.
On the other hand, infrared sensors cannot replace the function of radar; rather, radar and infrared systems complement each other, For example, infrared radiation can be attenuated to unusable levels by clouds, fog, rain, snow, etc. while radar can operate effectively in such weather. In addition, many target/background combinations appear significantly different when viewed in different regions of the electromagnetic spectrum. Some targets are therefore more easily detectable in one region than another. Furthermore, information received in two or more spectral regions can often aid in identification and recognition of a potential target, rather than simply in detection. Thus, the use of several types of sensors in conjunction with each other can yield a much higher probability of mission success under a greater variety of circumstances than can the use of one mode or kind of detector operating individually.
Since space is always at a premium in packaging electromagnetic detection systems, particularly when the system is packaged in a missile, it is often impractical to consider the inclusion of separate sensors in a weapon delivery system. Separate optics, antennas and/or scanning systems would also undesirably result in high cost and weight.
Accordingly, it is an object of this invention to develop a multimode antenna for millimeter radar including an infrared sensor, both detectors utilizing a common aperture system.
It is further object of the invention to establish an electromagnetic scanning system which uses rotating prisms to direct the view of the detection system to selected target regions, said prisms being transparent to all modes of electromagnetic energy used in the antenna.